change serialization to use `uint8_t` instead of `char`

[Karsten1987]

The serialization payload should really be serialized with uin8_t instead of char to guarantee a consistent size of 8bit on all platforms. See here: https://github.com/ros2/rmw_iceoryx/blob/master/rmw_iceoryx_cpp/include/rmw_iceoryx_cpp/iceoryx_serialize.hpp#L33